The mechanisms that lead to a release of radioactive contaminants (venting) are discussed. The processes that appear to be of primary concern in dynamic venting are (1) uncontrolled cavity growth toward the free surface, (2) energy flow through large diameter openings, and (3) upward displacement of cavity gases during chimney formation. For Plowshare experiments, the first and the third become the controlling mechanisms for dynamic venting. Logically derived models based on a fundamental physical understanding of these mechanisms are presented. These models provide an initial step toward developing criteria which assure that dynamic venting through the ground will not occur.